1. Field of the Invention
The present invention relates generally to the field of orthogonal frequency division multiplexing (OFDM) communication systems, and more particularly to the field of frequency synchronization in OFDM communication systems.
2. Description of the Related Art
Orthogonal frequency division multiplexing (OFDM) is a robust technique for efficiently transmitting data over a channel. This technique uses a plurality of sub-carrier frequencies (sub-carriers) within a channel bandwidth to transmit the data. These sub-carriers are arranged for optimal bandwidth efficiency compared to more conventional transmission approaches, such as frequency division multiplexing (FDM), which waste large portions of the channel bandwidth in order to separate and isolate the sub-carrier frequency spectra and thereby avoid inter-carrier interference (ICI). By contrast, although the frequency spectra of OFDM sub-carriers overlap significantly within the OFDM channel bandwidth, OFDM nonetheless allows resolution and recovery of the information that has been modulated onto each sub-carrier. Also, OFDM is much less susceptible to inter-symbol interference (ISI) from the use of a guard time between successive bursts.
Although OFDM exhibits several advantages, prior art implementations of OFDM also exhibit several difficulties and practical limitations. The most important difficulty with implementing OFDM transmission systems is that of achieving timing and frequency synchronization between the transmitter and the receiver. In order to properly receive an OFDM signal that has been transmitted across a channel and demodulate the symbols from the received signal, an OFDM receiver must determine the exact timing of the beginning of each symbol within a data frame. Prior art methods utilize a xe2x80x9ccyclic prefix,xe2x80x9d which is generally a repetition of part of a symbol acting to prevent inter-symbol interference (ISI) between consecutive symbols. If correct timing is not known in prior art receivers, the receiver will not be able to reliably remove the cyclic prefixes and correctly isolate individual symbols before computing the FFT of their samples. In this case, sequences of symbols demodulated from the OFDM signal will generally be incorrect, and the transmitted data bits will not be accurately recovered.
Equally important but perhaps more difficult than achieving proper symbol timing is the issue of determining and correcting for carrier frequency offset, the second major aspect of OFDM synchronization. Ideally, the receive carrier frequency, f.sub.cr, should exactly match the transmit carrier frequency, f.sub.ct. If this condition is not met, however, the mismatch contributes to a nonzero carrier frequency offset, DELTA.f.sub.c, in the received OFDM signal. OFDM signals are very susceptible to such carrier frequency offset which causes a loss of orthogonality between the OFDM sub-carriers and results in inter-carrier interference (ICI) and a severe increase in the bit error rate (BER) of the recovered data at the receiver. In general, prior art synchronization methods are computationally intensive.
Accordingly, there is an existing need to provide alternatives to synchronization in OFDM communication systems. More particularly, there is an existing need to provide alternatives to frequency synchronization that are less computationally intensive than the prior art.
Methods and apparatus for use in obtaining frequency synchronization in a multicarrier modulated system utilizing a frequency band of orthogonal narrowband carriers are described. The frequency synchronization described herein relates to the use of a coarse frequency correction process, a fine frequency correction process, and an overarching iterative process that makes use of both the coarse and fine frequency correction processes.
The iterative method involves the steps of performing a coarse frequency correction process which is operative to adjust receiver frequency so that a pilot tone signal is within a predetermined frequency range and, after performing the coarse frequency correction process, performing a fine frequency correction process which is operative to adjust receiver frequency so that the pilot tone signal is substantially aligned with a pilot tone reference within the predetermined frequency range. From performing the coarse frequency correction process, receiver frequency is adjusted so that the pilot tone signal is within the predetermined frequency range. However, the pilot tone signal may be outside a Nyquist sampling frequency range which undesirably causes an alias pilot tone signal to be within the Nyquist sampling frequency range. Assuming this condition, from performing the fine frequency correction process, receiver frequency is adjusted so that the alias pilot tone signal is substantially aligned with the pilot tone reference and the pilot tone signal is undesirably shifted outside the predetermined frequency range.
To eliminate any such result, the method further involves, after performing the coarse and the fine frequency correction processes, performing the coarse frequency correction process again and, after performing the coarse frequency correction process again, performing the fine frequency correction process again. From performing the coarse frequency correction process again, receiver frequency is adjusted so that the pilot tone signal is within both the predetermined frequency range and the Nyquist sampling frequency range. From performing the fine frequency correction process again, receiver frequency is adjusted so that the pilot tone signal is substantially aligned with the pilot tone reference.